To date, a magnetic disc storage device has been commonly used as an auxiliary storage device for information appliances like a computer. In a magnetic disc storage device, data read-out/writing operations are performed for each of storage units which are referred to as “sectors”.
In recent years, the substitution for the magnetic-disc storage device as described above, there has been a considerable increase in the alternative employment of storage devices which use semiconductor memories as their storage media. Of these storage devices, a storage device which uses a flash memory as its storage medium is gradually becoming the mainstream of this field at present. Here, the flash memory is a kind of the EEPROM (Electrically Erasable Programmable Read-Only Memory), i.e., the electrically-erasable and rewritable non-volatile semiconductor memory.
Such a flash memory is superior to the magnetic-disc storage device in its data read-out/writing speeds. In association with the use of this flash memory, however, there exist the following four limitations: The first limitation is as follows: The unit of read-out/writing data (which, generally, is referred to as “a page”) and the unit of erasing data (which, generally, is referred to as “a block”, and which is constituted from a plurality of pages) are determined and fixed. Accordingly, the block is larger than the page in size. The second limitation is as follows: When overwriting data, the data needs to be rewritten again after erasing the data once temporarily. The third limitation is as follows: When writing data into a certain page in a block, the data needs to be written in accordance with the sequence of continuous page numbers. The fourth limitation is as follows: The upper-limit of the erasing number-of-times is determined and fixed on each block basis.
In JP-A-2011-070470, the description has been given concerning the invention of a storage device using the above-described flash-memory. To achieve a storage device using a memory system which flexibly meets a need of a user, in JP-A-2011-070470, the disclosure is given as follows: “The semiconductor memory device according to the present invention includes: an NAND flash; a user area which has stored user data and is accessible from the outside; a system area which has stored the system information of the NAND flash and is an area exclusive from the user area; a DRAM with the user area and the system area; an NAND-CTL for performing access to the NAND flash; a DRAM-CTL for performing access to the DRAM; an internal bus 16 arranged between the NAND-CTL and the DRAM-CTL for enabling the NAND-CTL to access the system area; and a memory controller with the NAND-CTL, DRAM-CTL, and the internal bus.”